This invention relates to a process for artificially aging wire produced from rod of aluminum alloys that can be artificially aged, after working, to precipitate one or more constituents for development or enhancement of desired properties. It is particularly applicable to alloys containing magnesium and silicon (i.e. Al-Mg-Si alloys), wherein the precipitated constituent is magnesium silicide (Mg.sub.2 Si). In an important specific sense, the invention is directed to a process for producing artificially aged 6201 alloy electrical conductor wire.
For purposes of illustration, the invention will be particularly described with reference to wire made of the alloy having the Aluminum Association designation 6201, viz. an alloy of the following composition:
______________________________________ Range or Maximum (%) ______________________________________ Si 0.50-0.9 Fe 0.50 Cu 0.10 Mn 0.03 Mg 0.6-0.9 Cr 0.03 Zn 0.10 B 0.06 Others 0.03 each/0.10 total Al balance ______________________________________
In at least many commercial electrical conductor applications, it is required that the produced 6201 alloy wire be in the condition known as T81 temper and have the conductivity prescribed by ASTM specification B-398. That is to say, the wire is required to have an elongation (in 10 inches) of at least 3.0%, an average tensile strength of at least 48,000 p.s.i. (46,000 p.s.i. minimum) for wire of diameter less than 0.1327 inch, and 46,000 p.s.i. average (44,000 p.s.i. minimum) for wire of diameter of 0.1327 inch and above, and a conductivity of at least 52.5% of the International Annealed Copper Standard (IACS).
Such wire, in present-day commercial practice, is commonly produced by drawing down solution-heat-treated 6201 alloy rod in a so-called drawbench having a succession of dies through which the workpiece is drawn under tension (while oil is applied) to effect progressive reduction in diameter. For example, 3/8-inch diameter rod may be drawn down in this manner to produce wire of any desired diameter within a range of 0.060 inch (or below) to 0.188 inch (or above). At the exit end of the drawbench, the wire is wound on spools or bobbins.
For commercial production of 6201 alloy wire in T81 temper with ASTM B-398 conductivity, the wire must be subjected to artificial aging. Although such properties may be developed without artificial aging over extremely long time periods (months or even years) by "natural aging," i.e. very extended ambient-temperature storage of the wire, these time period are so long that natural aging is utterly useless in any practicable sense, and may be disregarded as of no consequence.
Heretofore, the artificial aging process has involved placing hot or cold bobbins of the wire in a heated oven for a prescribed time, in either a continuous aging operation (with conveyor transport of the bobbins through the oven) or a batch aging operation, and with oven temperatures ranging from 275.degree. F. to 320.degree. F. depending on cycle time and oven type. The term "oven aging" is used herein to refer to procedures wherein heat is supplied (in an oven or the like) to wire after drawing for the purpose of precipitating constituents to enhance desired mechanical properties and/or conductivity. It has been believed that these oven agings were necessary to artificially age the material by precipitation of magnesium silicide--as required to attain the desired mechanical properties and conductivity--without starting recrystallization in the aluminum matrix. Rapid precipitation begins from 305.degree. F. to 320.degree. F., depending on the heat history of the rod and drawn wire.
The described oven aging is disadvantageous in that it necessitates provision of aging furnaces or ovens, with attendant capital cost, and also requires supply of additional energy after drawing, as well as involving performance of a post-drawing step requiring inconveniently close control of process conditions. Thus, for the sake of convenience and economy, it would be desirable to produce an artificially aged 6201 wire product without oven aging, yet with reliable attainment of the requisite conductivity and mechanical properties. It would also be desirable to reduce scrap (due to tensile and conductivity rejections) as compared with conventional oven aging, where (for example) excessive temperatures and/or times may result in "overaging" with consequent loss of tensile strength.
Although discussed above with specific reference to 6201 alloy, the foregoing considerations are likewise applicable to other alloys of the 6000 series; and more generally, in the production of wire by drawing down rod of aluminum alloys such as Al-Mg-Si alloys, it has been customary to subject the wire to oven aging after drawing for precipitation of constituents and consequent enhancement of mechanical and/or conductivity properties. It would be desirable to achieve these properties without the capital cost, energy requirements, and inconvenience of the conventional oven aging step.